In recent years, semiconductor lasers have been used in many fields. For example, semiconductor lasers of red, green, and blue that are three primary colors of light have been all achieved; therefore, it is expected that the semiconductor lasers take advantages of characteristics such as a small size and low power consumption to be applied to image display units such as televisions and projectors. At the same time, for the purpose of application expansion, a further improvement in characteristics of the semiconductor lasers is demanded.
At present, the semiconductor lasers of green and blue of the three primary colors mainly use a hexagonal nitride semiconductor. In the nitride semiconductor, a band in a well layer has an inclination by spontaneous polarization or piezoelectric polarization. This causes an issue that electrons and holes are spatially separated to decrease light emission intensity. Such separation of electrons and holes by spontaneous polarization or piezoelectric polarization is observed not only in the nitride semiconductor but also in, for example, semiconductors having a sphalerite structure such as InGaAs/GaAs-based semiconductors.
In order to solve this issue, for example, PTL 1 discloses a structure in which a band gap is continuously inclined in a well layer. PTL 1 describes that the band gap is inclined toward a direction where a band inclination by piezoelectric polarization of a conduction band or a valence band is reduced, thereby reducing spatial separation of electrons and holes to improve light emission intensity. Moreover, in PTL 1, such an effect is achieved more remarkably in a case where the inclination of the band gap is increased.